A library device that holds a large number of cartridges is known. The library device has a configuration in which a plurality of cartridges, each holding a recording medium, and drives are mounted in a rack. The library device includes a robot for retrieving a cartridge and loading the cartridge into a drive. The robot moves to the location of a target cartridge, unloads the cartridge, then moves to the location of a target drive, and loads the cartridge into the drive.
FIG. 36 illustrates an example of a library device according to a typical related art. Cells 302, holding cartridges 501, and drives 402 are installed in a rack of a library device 101 illustrated in FIG. 36. A robot, not depicted, takes a cartridge 501 from a cell 302 and loads the cartridge 501 into a drive 402.
Another example of a library device is described in Japanese Laid-Open Patent Publication No. H07-021644. In the library device described in Japanese Laid-Open Patent Publication No. H07-021644, video cassette tapes are held in a rack. Grippers move in the width direction and the height direction of the rack and unload a video cassette tape. The rack is separated into two layers in the depth direction and video cassette tapes can be held in each layer in the depth direction. When unloading a tape from the back (the second layer), a first gripper pulls out a tape from the first layer at the front and a second gripper pulls out the target tape from the back (the second layer).
However, increasing the load capacity of the library device described above and illustrated in FIG. 36 requires extending the width of the library device, which results in a long travel distance of the robot. An increased travel distance entails increases in the lengths of cables for supplying power and signals to the robot. As a result, supply voltage and signal quality decrease, which can lower the stability of operation. Furthermore, since the whole width of the library device increases, the flexibility of installation is reduced.
In addition, in the technique described in aforementioned Japanese Patent Laid-Open Publication No. H07-021644, as many grippers as the number of cartridges held in the depth direction of the rack need to be provided and a cartridge in the front needs to be picked up before picking up a cartridge in the back. Accordingly, the technique described in Japanese Patent Laid-Open No. H07-021644 can only hold as many cartridges as the number of grippers in the depth direction of the rack. Moreover, as the number of cartridges held in the back increases, the number of pickup operations increases, thereby, consuming the time required for unloading and loading cartridges.